Monthly Archives: September 2020

Zinc Bromine Batteries: First tests using TMPhABr

As I’ve mentioned in previous posts, tetrabutylammonium bromide (TBABr) is not a very good sequestering agent for static Zn-Br batteries due to its very low solubility in Zinc Bromide solutions. To solve this problem, I have decided to test trimethylphenylammonium bromide (TMPhABr) as a potential replacement, since this salt also forms and insoluble perbromide but – due to its significantly higher polarity and lower molecular weight – should be significantly more soluble than TBABr. I ordered it from Alibaba around one week ago and recently got it delivered.

Picture of the TMPhABr I got from China

My initial tests with it involved testing its solubility in Zinc Bromide solutions. The solubility of TMPhABr in pure water is not indicated clearly anywhere, but I assumed its solubility would be similar to that of trimethylbenzylammonium bromide (TMBABr) or tetrapropylammonium (TPABr) bromide, both which have solubilities of around 10% by mass in water at 25C. My initial tests have confirmed this suspicion with solutions at 10% by mass being easy to prepare at 20-25C. I didn’t try to prepare more concentrated pure solutions as my objective is to judge its solubility in the presence of Zinc Bromide.

The first test I performed to evaluate this was a 0.25M solution of Zinc Bromide which was able to dissolve 0.12M of TMPhABr with no problems. I then increased the amount of ZnBr2 to 0.5M – which is what the authors of the Chinese paper using ZnBr2+TPABr use – and I was able to dissolve 0.25M of TMPhABr without issues. With this result I know I will be able to at least reproduce similar experimental conditions to those achieved by the Chinese researchers, something that I could never do with TBABr due to the solubility issues mentioned before.

To test how far I could take this I then attempted to prepare a 1M solution of Zinc Bromide and see if I could get 1M of TMPhAbr to go with it. Sadly at this point the concentration of TMPhABr is already too high – would be close to 10% by weight of the solution – so it was actually not possible to get to this point. This means that the practical limit of this battery will be to have around 0.25M of TMPhABr dissolved, which is probably a realistic limit for most quaternary ammonium salts since we are unlikely to get an effective sequestering agent – not electrochemically active and with no effect on pH – with a molar mass significantly lower than that of TMPhABr at a similar price point.

First two charge/discharge curves measured (at 2mA constant current). Battery was charged to 500 uAh and then discharged to 0.5V. First curve, CE=68%, EE=57%. Second curve, CE=79%, EE = 66%.

I then used this 0.5M ZnBr2 + 0.25M TMPhABr solution to create the first battery. This battery had a diameter of 0.5 inches and was built within my Swagelok cell. I used a 0.2mm thick Zinc anode followed by 8 layers of fiberglass separator and a CC4 carbon electrode. I also made sure to sand the graphite electrodes in the Swagelok cell to make sure their exposed surface was pristine. I put 50uL of the electrolyte on the cell but I won’t know how much ended up in the separator until I open the cell after testing and weight the wet components.

The graph above shows the first – to the best of my knowledge, the first ever public – charge/discharge curves of a static Zn-Br cell prepared using TMPhABr as a sequestering agent. It is very interesting to note that the shape of the discharge curve improved immensely moving from TBABr, showing that this battery is significantly better behaved. Although the CE and EE of this first curve were particularly low, the CE of the second curve measured already showed an increase of the CE to 79% and EE 66%. I will keep cycling the battery and will show you how the CE and EE change as a function of the number of cycles. Exciting times!

Zinc Bromine Batteries: What would be realistically required?

Current commercial Zn-Br flow batteries have specific energies in the 34.4–54 W·h/kg region, with most companies being at the lower end of this range. In order for a static Zn-Br battery to be better than its current industrial counter-parts it would ideally improve on this specific energy while reducing the costs of production substantially.

My current tests using carbon cloth cathodes, Zinc anodes, fiberglass separators and Zinc Bromide electrolytes in the 0.25-0.5M range with a TBABr sequestering agent present at concentrations of around 0.1-0.2M have shown an ability to store around 0.5mAh with a weight of around 0.250g per total cell (no packaging material), which would give the cells a specific energy of around 3 W·h/kg, which is one order of magnitude lower than current commercial Zn-Br flow batteries.

An image of one of my current Zn-Br battery cells. The cell has a diameter of 0.5 inches and is placed inside a Swagelok cell with graphite electrodes for measuring

In terms of weight, I have been using a 0.2mm thick Zinc anode that is quite thicker than what would be strictly necessary for the battery, the anode thickness can be changed to 0.02mm Zinc foil (10x less mass) which would reduce the total amount of mass by more than 70%. The anode mass is currently around 180mg, so lowering this to 18mg would take the current specific energy to around 9 W·h/kg (since there is no expected loss in the current battery configuration from using a thinner Zn anode).

This improvement is still not enough, we need to increase the capacity by at least 4-6x which means increasing the amount of Zinc Bromide in the battery to at least the 1.5-2M range and increasing the amount of energy injected/extracted to at least 2.0-3.0 mAh for this battery. This means that TBABr is not going to work, reason why my tests are now going to move to using TMPhABr or TPABr. These new sequestering agents also have lower molecular weights, so they are bound to be significantly more “atom efficient” compared to TBABr. The end batteries right now contain around 50uL of electrolyte – I put 100uL but half is “pushed out” when Swagelok cells are closed (this is determined by weighting the dry and final battery cell) – so theoretically a 2-4M Zinc Bromide solution should offer a capacity of around 2.7-5.2 mAh but we are unlikely to be able to extract this amount because of the conductivity of the solution becoming lower as we plate Zn and oxidize bromide to perbromide in the cathode.

The current energy efficiency of the battery is still too low (max has been 60% in most cases) so the hope is that the higher Zinc Bromide concentration, coupled with the new sequestering agents, will help increase this efficiency to the 70-80% region while also helping improve maintain Coulombic efficiencies above 95%. The energy efficiency of current Zn-Br flow batteries is mostly below the 80% mark, so anything above this number would be highly desirable.

If the above mentioned sequestering agents can achieve these efficiencies at these concentrations then we would be able to reach specific energies of around 45 W·h/kg for the cells I’m constructing. If we can achieve energy efficiencies above 90% – already seen in published research using TPABr – this would already put them at a significantly more competitive place relative to current Zn-Br technology.

Currently Li-ion cells are in the 100-265 W·h/kg range, so this technology could only compete if significantly higher zinc bromide concentrations – in the order of 10M – can be achieved, while retaining a functional sequestering agent or if we can add a supporting electrolyte that enables the extraction of most of the zinc bromide without lowering the efficiency of the battery (although that electrolyte adds some weight). It is much more likely that a technology like this would compete in battery life and USD/kWh terms. Li-ion technology right now is at around 200 USD/kWh while a technology like Zn-Br in static cells could start at a fifth of this price. The life of a static Zn-Br battery with a viable sequestering agent is also expected to be significantly longer (>10,000 cycles) so that would also help it compete with Li-ion (with Lithium Iron Phosphate batteries surviving for around 2000 cycles when fully discharged on each cycle).

Zinc Bromine Batteries: Can we just put solid TBABr in there?

I have mentioned how the usability of TBABr in Zn-Br batteries is limited due to the poor solubility of TBABr in the presence of large concentrations of zinc bromide. In my experiments the most concentrated solution I was able to get was around 0.1M ZnBr2 + 0.1M TBABr. This is problematic since we aren’t going to achieve high specific energy or power values with an electrolyte that is this dilute in terms of Zn concentration. However, what if we put a suspension into the cell as the electrolyte?

Image of the TBABr + ZnBr2 suspension prepared

When the cell is charging, the concentration of TBABr in the electrolyte will go down as TBABr3 precipitates out of solution. However, if there is extra TBABr within the cell, that solid will dissolve to replace the TBABr that precipitated. When the cell is discharged, the process will reverse, TBABr3 will redissolve and some TBABr will precipitate again as it is pushed out of solution by the perbromide that needs to go back into solution. The conductvity of the solution should be less affected, because it will only be reduced as a function of the loss of ZnBr2, without an actual loss of TBABr. The problem of course, is that there will be some solid TBABr in the cell, which is likely to increase the series resistance of the cell (because the solid salt is not a conductor).

How do we achieve this? To do this I first put 0.720g of ZnBr2 into a 10mL volumetric flask, then dissolved that into 1mL of distilled water. I then added as much 1M TBABr solution as needed to fill the volumetric flask to 10mL. The total concentration of ZnBr2 is around 0.33M but a lot of “solid” precipitates out of solution, forming a high viscosity phase with the consistency of honey that is made almost entirely out of TBABr. If we agitate the flask, this phase gets suspended into solution quite easily, forming a cloudy suspension (see image above).

Evolution of CE and EE as a function of the cycle number. The cell was charged to 500 uAh and discharged to 0.5V, both at 1mA.

I then built a battery within my graphite electrode Swagelok cell using a zinc anode, 8 layers of fiber glass separator and a carbon felt cathode. I then added 100uL of the above prepared suspension right after agitating the flask vigorously, allowing the material to wick through the cell for a minute before closing the Swagelok cell.

I have since started doing charge/discharge cycles of this cell with very interesting results (see above). The cell initially had relatively low coulombic efficiency (CE) and energy efficiency (EE) values, but these started improving as the cell was cycled. My hypothesis is that – per my previous explanation – the solid is first randomly distributed within the cell but gets organized and deposited within the cathode as the number of charge/discharge cycles increases. I believe this greatly improves the formation of the TBABr3 within the cathode and prevents the solubilization of the perbromide, which reduces self-discharge and therefore increases the cell’s efficiency.

All charge/discharge curves for the cell up until now.

I believe we can see some experimental evidence for this hypothesis as we see a “shoulder” emerge at the start of the charge phase as the number of cycles increases. I think this is consistent with a significant amount of TBABr deposited close to the cathode interface after discharge, which creates a higher resistance to current flow that subsides as the TBABr3 starts forming and this TBABr dissolves back into solution. This is of course an interpretation based on very limited information and I would be thrilled to know what any of you think about the evolution of the charge/discharge curves and what you believe they are telling us. I will continue cycling this cell during the next 2-3 days, to see how the cell stabilizes and whether the CE and EE start going down after.

With that said, it seems pretty clear that TBABr by itself is not going to be an adequate sequestering agent. I will be trying to use PEG200 to increase its solubility – as discussed in some of my previous posts – but I also already ordered TMPhABr (trimethylphenylammonium bromide) as I believe this will be a way better sequestering agent for these devices.

Zinc Bromine Batteries: How can we increase the solubility of TBABr?

As I mentioned in a previous post, the most important issue with the use of tetrabutylammoniumbromide (TBABr) in static Zn-Br batteries, is that the solubility of TBABr drops very sharply when zinc bromide is also in solution. While you can prepare 50% w/w solutions of TBABr in distilled water, the max concentration drops to around 0.15M when preparing solutions in the presence of 0.5M of zinc bromide. This is very bad because – in order to function as an effective sequestering agent – we would want the concentration of TBABr to be able to be significantly higher in solution.

Tetra-n-butylammonium bromide - Wikipedia
Graphic representation of the TBABr salt. You can see that the TAB+ cation has a strong aliphatic component

The solubility of TBABr drops because of a sharp increase in the polarity of the solution due to the introduction of the Zn+2 ions, which are small and – due to their double charge – substantially increase the dielectric constant of the medium. The TBA+ cation is actually not that polar, being spherical and with a strong aliphatic component, meaning it cannot very successfully interact with this new, much more polar medium. As a consequence the TBABr drops out of solution.

In order to prevent this from happening, we need to find solutions that either make the Zn cation less polar or make the media less polar by introducing a less polar additive that can compensate for the increase in polarity brought by the Zn cation. These two potential solutions however, need to avoid the TBABr3 becoming soluble as the perbromide needs to remain insoluble for the battery to work as designed (create an insoluble perbromide to prevent self-discharge).

To make the solvent less polar by adding something else, we need to consider our potential choices and their polarity. We could add another solvent that doesn’t react with perbromide, like an alcohol, but we would need to be very careful with the amount to ensure that it does not make the perbromide soluble (since we know TBABr3 is slightly soluble in alcohols (see here)). We could also decrease the polarity by adding a polymer – like PEG 200 – which also has the benefit of decreasing the formation of dendrites in the Zinc anode. Both of these solutions are potential avenues for experimentation.

Zn(II)-EDTA | Dojindo
The EDTA complex formed between EDTA and Zinc ions

To decrease the polarity by masking the Zinc ion we can use a chelating agent that can react with the Zinc in order to reduce its effect on the dielectric constant of the medium. We could do this by replacing ZnBr2 with ZnEDTANa2 which replace bromides by the Zn(EDTA)-2 complex and requires the addition of two sodium ions, which are bound to be significantly less polar than the Zn+2 cation. However this would imply we would have less bromide available, so it might require the addition of NaBr to recover the equivalent moles of bromide we have lost. Alternatively we can also just add NaH2EDTA2 but we would require to make pH adjustments to the electrolyte, which is not something we would like to do. Additionally, the ZnEDTANa2 reagent is cheap and easily available – as it’s used as a fertilizer in agriculture – and the NaBr is also really low cost. This solution decreases the specific energy/power of the battery though, as the weight is increased by the use of additional reagents.

So there you have it, three potential experiments to try to make TBABr a viable sequestering agent for high energy/power density Zn-Br static batteries. Will they work? I plan to test them out one by one!

Zinc Bromine Batteries: Is TBABr the best complexing agent?

Secondary Zn-Br batteries suffer from a huge problem of self-discharge due to the formation of elemental Bromine which, although largely insoluble in water, is soluble enough to migrate through the cell and react with the zinc anode, effectively self-discharging the cell.

To circumvent this issue, researchers have used chemicals that sequester the produced bromine into a product that has even less affinity for water — an insoluble or immiscible perbromide. In flow batteries this is done to generate a liquid phase that is immiscible with water, since it still needs to be a liquid to allow proper flow of the reagent. In static batteries this is undesirable, because a liquid is still able to flow through the cell and react with the Zn anode.

This is a figure taken from the Chinese paper. You can see that they do test the TBABr for its perbromide’s solubility

The 2020 Chinese paper we’ve discussed previously in this blog goes around this problem by using a sequestering agent that forms an insoluble perbromide, tetrapropylammonium bromide (TPABr). Notably the paper uses TPABr instead of tetrabutylammonium bromide (TBABr) which is almost an order of magnitude cheaper due to its significantly wider array of industrial uses compared to TPABr. Not only that, but the TBABr perbromide is even more insoluble, so the chemistry should be even better, right?

It is worth noting that they are aware of the above facts. You can see this in the image above – taken from the supporting information of the paper – where they clearly show TBABr forms an insoluble perbromide. So why did they choose to go with a significantly more expensive chemical (TPABr) and not use TBABr when its the obvious choice from a practical standpoint?

Precipitation of TBABr from a completely transparent TBABr 1M solution when in contact with a 0.5M Zinc Bromide solution

The problem – which I have lived through experimentally – is that the solubility of TBABr in the presence of ZnBr2 is quite terrible. The TBABr is extremely soluble in water – you can easily prepare a 50% solution by weight in distilled water – but it precipitates back very aggressively when put it into contact with a solution of zinc bromide. The image above shows you what happens when you mix a 1M solution of TBABr with a 0.5M solution of ZnBr2. The authors of the paper probably saw this issue and immediately recognized this as a potential problem for their batteries, my intuition is that they did run and have results for some cells using TBABr, but the results were probably so much worse than those of TPABr, due to this solubility issue, that they simply did not publish them.

The TPABr is most probably a significantly better sequestering agent because it’s likely significantly more soluble than TPABr in Zinc Bromide solutions. This agent is however unlikely to be soluble enough to support very large capacity solutions (>= 2M ZnBr2).

As I mentioned on a previous post, a better sequestering agent must allow for large solubility, be commercially available and form an insoluble perbromide. The only candidate I can think of to fulfill this role would be trimethylphenylammonium bromide (TMPhABr). I might be tempted enough to test it to order some from Alibaba if I can get a low quantity order for a reasonable price!

Zinc Bromine Batteries: Initial thoughts about a practical battery

During this past week I have been experimenting and thinking more about Zn-Br batteries and how a real practical battery would look like (how it would be built and what its characteristics would be like). Let’s imagine we have found a complexing agent that can be used in highly concentrated ZnBr2 solutions. What would a prototype battery look like and how much would it cost?

The first thing we need to consider is the geometry to build such a battery. Single-cell batteries for Zn-Br chemistry are impractical due to the limits that would impose on current density – and it’s not the 19th century – so the ideal battery would probably follow a configuration similar to modern lead-acid batteries, where multiple cells are put together to achieve better results. The simplest way to do this is to stack materials next to each other within a box, then flood the box with the desired electrolyte solution.

Proposed stacking of layers for a battery built in a 101x54x55mm project box. Note that the cells are laid horizontally (left to right) . Fiberglass separator thickness should be increased so all contents fit tightly inside the box.

In a 101mm x 54mm x 44mm project box you could fit a volume of around 237mL. If we decide to use a very porous carbon felt electrode – which I have experience with – with titanium current collectors, glass fiber separators and zinc anodes, we would create a cell configuration like the one showed above. This would occupy the entire cell with either separator, current collector, anode or cathode material. Given that the materials used take little real volume, as they are either very porous or very thin, I’m going to assume the solution volume we will fit will be equal to 200mL, which is realistic given the characteristics of the materials.

If we use a 2M ZnBr2 solution, this would give a maximum theoretical energy of 40Wh. If the cells are all connected in parallel, this would give us a battery with a voltage of 1.85 at a capacity of 21.6 Ah. The battery would be charged at a constant current of around 2.85A, although depending on the actual kinetics this might need to go down to even 285mA. In the above design you actually have only 6 cells that are each equal to 2 normal cells connected in parallel – as they share a current collector in the cathode – so connecting these 6 in series would give you a voltage of 11.1V with an expected charging current of 475mA.

The main caveat of the above design is that it uses a 2M ZnBr2 solution, assuming we can find a complexing agent that forms an insoluble perbromide that can be in the initial formulation at a concentration equal to at least the same as the ZnBr2 then this should be no problem. After a lot of research about the solubility of perbromides and organic ammonium salts I believe this might be possible using trimethylphenylammonium bromide, but such a complexing agent has never been tried! The 200 mL of solution used here would use 90.07g of ZnBr2 and 86.45g of TMPB.

Note that this configuration would certainly not work without a complexing agent that precipitates the tribromide formed. Without it the bromine would pool at the bottom and discharge the cell – in a horizontal configuration – or just sink and discharge the cell in a vertical configuration.

Cost (USD)ItemURL
3Project boxhttps://www.allelectronics.com/item/mb-132/abs-project-box-3.97-x-2.12-x-1.72/1.html
45Carbon felthttps://www.ceramaterials.com/product/gfe-1-pan-graphite-felt/
10.99Fiberglass tissuehttps://www.amazon.com/Multipurpose-Fiberglass-reinforcing-waterproofing-membranes/dp/B0719KWMJ7/ref=sr_1_5?dchild=1&keywords=fiberglass+paper&qid=1600038562&sr=8-5
11.49High purity Znhttps://www.amazon.com/99-9-Sheet-Plate-Metal-140x140mm/dp/B086FGDW83/ref=sr_1_5?dchild=1&keywords=Zn+sheet&qid=1600038696&sr=8-5
9.01Zinc BromidePrice with shipping confirmed from Alibaba vendor
19.99Titanium foilhttps://www.amazon.com/0-3mm-200mm-300mm-Titanium-Purity/dp/B07G8YYPFV/ref=sr_1_2?dchild=1&keywords=titanium+foil&qid=1600040029&sr=8-2
18.85TMPBPrice with shipping confirmed from Alibaba vendor
Potential materials used to construct a prototype Zn-Br cell

Using all the materials above, the cost of building such a prototype would be in the order of probably 120 USD. Probably around 200 USD after you add shipping for everything. In reality this cell is also unlikely to yield 40Wh and will most likely be in the vicinity of 20Wh if everything works as expected.

It is also important to note that an ABS project box like the one above is a risky first-choice, given that ABS can adversely react with elemental bromine, so a PTFE project box would – although much more expensive – be a safer choice for a prototype. By the time I build something like this, I hope I have already established that TMPB forms insoluble enough perbromide salts under my much more controlled Swagelok cell conditions.

Note that I am still far away from executing something like this! Currently I am even far away from testing a TMPB cell, but I wanted to write this blog post to condense all this theoretical research and serve as a referring point for me or others in the future.

Zinc Bromine Batteries: Can they really be that good?

In my quest to study Zinc-Bromine batteries, I have been diving deep into this 2020 paper published by Chinese researchers, which shows how Zn-Br technology can achieve impressive efficiencies and specific power/capacity values, even rivaling lithium ion technologies. I’ve found some important things when studying this paper, that I think anyone looking into this technology should be aware of.

An example of specific capacity values for different cells measured by the Chinese researchers.

First, let’s talk about the specific capacity values found within this paper. Usually the cells in the study would be charged to a specific capacity of around 500 mAh/g, with Coulombic efficiencies greater than 99% in some cases. The cell used has a diameter of 0.5 inches – which gives us an area of around 1.29032 cm^2 for the electrodes – with the capacity per area at 1.5 mAh/cm^2. This is pretty amazing, because it means we are getting 1.5 mAh out of 3 mg of active material. Wait, what?

The researchers are pretty clear in mentioning that they calculate the specific capacity values given the weight of the cathode but, what they fail to mention explicitly, is that this is not the weight of the entire cathode material but merely the conductive active material within the cathode, which has a carbon loading of 3mg per cathode piece. The real cathode does not weight 3mg, it weights significantly more – probably an order of magnitude more – given that the cathode is prepared using a binder in an 8:1:1 proportion with the carbon sources. This also does not count the weight of the electrolyte or the weight of the anode.

Granted, the above approach is not uncommon in battery research – reporting only capacity values of active materials – but in this case, in the real world, it’s not like you’re going to be getting 250 mAh/g of battery, you’re likely going to be getting a lot less. In the case of the specific power/energy values the researchers take into account the mass of electrolyte and complexing agent (ZnBr2 and TPAB) but they do not account for the mass of the separator, anode, or other materials.

Specific power/energy values published on the paper

The unfair and potentially misleading part about this is that they are comparing a very partial weight of their Zn-Br system, with the actual values for specific energy and power that have been measured for actual production Li-ion systems. We know of commercial Li-ion systems with specific energy values of 150-250 Wh/kg where that is delivered per actual kilogram of actual battery (including packaging and everything else).

Realistically a Zn-Br fully built battery is likely to have a specific energy way lower than what is published in this paper. So – not very surprisingly – realistic battery systems built with this technology will likely have an energy and power more on the low end of what current Li-ion technology has to offer, although they are bound to be superior to current Zn-Br flow battery designs.

The paper and Zn-Br technology are still extremely interesting – at least to me – because of the high efficiencies, way lower discharge rates, higher specific energy/capacities and other advantageous properties over traditional Zn-Br flow batteries, but they are unlikely to be a game-changer in the energy industry. As most of the time, researchers want to make their numbers look as good as they can within the general practices of the field and battery research is not the exception. I’m not saying that the researchers are being unethical or lying, just that the reader must be aware of how researchers report these numbers and how they compare to what you actually get in final products.

With that said, the confusion from how specific capacity/energy/power are measured in battery research is not without controversy. With some important efforts going on (see here) to try to create clearer standards within the field to avoid confusion.

Zinc Bromine Batteries: Think about the electrodes!

In order to study the chemistry of Zinc-Bromine batteries I have been using a swagelok cell that I bought from China that has a central Teflon body with stainless steel electrodes. This has been problematic due to the reactivity of these electrodes with the elemental bromine and tribromide salts produced in the battery, requiring the use of some “improvisation” in order to make the batteries work.

Charge/Discharge curve of a Zn-Br battery built using a zinc anode, carbon felt cathode, fiber glass separator and 0.5M ZnBr2+ 0.2 TBAB electrolyte in a swagelok cell with stainless steel electrodes covered with conductive HDPE.

To be able to generate the necessary chemical reactions without interference from the stainless steel I have coated the electrodes with some conductive HDPE I have, which has a relatively high volume resistance of around 10K ohm. The charge/discharge curve above shows you the type of measurements I have been able to achieve with this setup, with the above curve having a Coulombic efficiency of 88%.

This is lower than what I could achieve with the copper anode I was using previously (which gave me around 96%), but note how the charging voltage is lower and the discharge voltage higher, meaning that the overall energy efficiency is significantly better (around double). This is however not because of the zinc anode, but because with the conductive HDPE now covering both electrodes, I have now been able to tighten the cell more and achieve a lower overall internal resistance.

However the still relative low energy efficiency and voltage drop when going from charge to discharge are still pointing to significant sources of efficiency loss, possibly from the 10K ohm resistance that the conductive HDPE is giving. The weird shapes at the beginning and end of the discharge curve are also pointing to more than one chemical reaction happening, probably because the electrodes are still somehow interfering with the chemistry (maybe some micro holes in the conductive HDPE at points of stress are exposing the electrodes underneath). For this reason I have decided to get graphite electrodes for the Swagelok cell, which I will build from these 0.5 inch diameter graphite rods I found.

I am also going to change the current carbon felt cathode for carbon paper electrodes – which are on the way – but I will refrain from using the paper until I can perform some tests using actual graphite electrodes that are guaranteed to be free of any pesky side reactions, with way less resistance than this conductive HDPE.

Zinc Bromine Batteries: Current battery and experiments to follow

This week I published a post about my first success in the making of a Zinc Bromine battery, this first battery had a Coulombic efficiency of at least 96% and was able to show the expected charge/discharge curves, which I hadn’t been able to see before. In this post I want to talk about some of the problems I have found and the experiments that will follow to attempt to fix them.

Current structure of my battery. The cell also includes around 80-100uL of a 0.5M ZnBr2+0.2M TBAB solution.

The structure of my current battery is shown above. The first problem I have run into are side reactions due to my use of copper tape as the anode used for zinc plating in the batteries. When I discharge the battery I seem to inevitably get some Cu oxidized and into solution, which is affecting the chemistry of the battery as a function of time. This means that I am losing a lot of coulombic efficiency and my charge/discharge curves are starting to show unwanted side reactions. I will be trying to replace this copper tape anode with a conductive HDPE covering plus a zinc anode to prevent any of these side effects.

The second problem comes from the use of a conductive carbon felt cathode that is pretty heavy (500mg per electrode used in the Swagelok cell) which means that my specific capacity is currently in the 0.5-1 mAh/g of cathode material, when ideally I should be seeing specific capacities in the order of 100-500mAh/g. The battery is already very efficient at using the electrolyte though as the maximum theoretical capacity of it is in the 0.01mAh/uL, given how much zinc and TBAB there is inside of it.

I have ordered an assortment of carbon paper materials (see it here) so that I can test whether these will offer me equivalent power storage with a significantly lower mass. I also ordered the MGL 190 carbon paper (see here) which seems especially promising given that I will be able to build a cathode weighing just 11mg for this area. This should allow me to reach much higher specific capacities if I’m able to sustain the same total capacity for the cell.

When I fully open the cells after going through a full charge cycle I do not observe any accumulation of yellow TBAB tribromide within the interior of the carbon felt electrode. This is telling me that whatever storage is happening is probably only going on within the first few microns of the cathode materials, meaning most of the cathode materials is actually being wasted and not being used for charge storage.

This is the new battery structure I’ll be moving to this week after I get the zinc anode and carbon paper materials.

Another problem with the carbon felt is that it has a lot of “loose hairs” that “sneak” into the porous fiberglass separator and cause shorts between the anode and cathode unless I use 4-6 layers of fiberglass I use (which is sadly pretty porous). This substantially increases the internal resistance of the battery and the hairs, although shorting the battery to a much lesser degree, may still be causing an incredible amount of self-discharge given that they do provide significantly shorter paths between the battery anode and cathode materials.

Getting rid of all copper, changing to a zinc anode, covering both anode and cathode with conductive HDPE and changing from a carbon felt cathode to a carbon paper cathode may all be moves that should help me greatly increase the performance of this battery. Stay posted for some further updates!

Zinc Bromine Batteries: First success!

In my first article about zinc-bromine batteries I discussed why these batteries are gaining interest and how some recent articles point to their potential use as reliable and cheap batteries, especially for large scale applications. After building my own DIY potentiostat/galvanostat, I wanted to use this technology to characterize home-made zinc-bromine batteries and experiment with their chemistry.

One of my initial attempts at a Zn-Bromine battery using carbon felt electrodes as both anode and cathode. Trying to charge the battery at 1mA/cm^2 never got above 1.32V and potential declined after time.

My previous article also mentioned some of my first attempts at building these batteries, which were mostly failed attempts due to the complexity of the battery builds. Even though I was able charge the batteries a little bit – and obtained relatively high Coulombic efficiencies when injecting a small amount of charge – I was never able to sustain potential values close to the expected 1.6-1.8V of the zinc bromine system. Always topping up at around 1.3-1.35V as shown in the image above, when trying to inject charges at 1mA/cm^2.

A huge problem of my first set of designs was a complete inability to adequately reproduce my batteries. The electrode construction was very complicated and every battery I tried had slightly different geometry and different amounts of electrolyte within their construction. In order to standardize the study I decided to change to a Swagelok cell construction (which I bought from China here). I bought a cell and got it delivered to the US within one week.

Button Cell Swagelok-Type Cell for Cell Testing
These are the Swagelok cells I am using to build my batteries now. These cells have an inner diameter of half an inch.

Although the Swagelok were supposed to make things easier, I started to face issues with the electrode material of the cells being reactive towards the bromine generated within the battery charging process. In my initial attempts using a carbon felt electrodes and a fiberglass separator, the stainless steel electrodes in the cell – which are inevitably exposed to the solution – were getting corroded away by the generated bromine and tribromine salts.

I was finally able to surmount these issues by covering the Swagelok cell electrode pieces with conductive HDPE, basically by wrapping the electrode with it and then inserting it within the Swagelok cell. Using this method I was able to produce my first successful Zn-Br cell using a tetrabutylammonium bromide (TBAB)/ZnBr2 solution (0.25 and 0.5M respectively) , a copper electrode for zinc reduction a fiber-glass separator and a carbon felt electrode for the tribromide depositing.

Charge/discharge curve of my first successful cell. I charged the cell to 500uAh and then discharged it until it reached 0.5V. This process was carried out at 1mA.

The image above shows you my first successful charge/discharge curve. To the best of my knowledge, this is the only example available online for experimental data of a TBAB/ZnBr2 cell. The Coulombic efficiency of the above cell was 96%, which is great considering this is the first successful one I have built. The cell used around 80-100uL of solution and 4 layers of fiber-glass separator (see my previous post for links to these materials).

I am still facing some issues related with the cutting of the separator/electrode materials to place within the cell (I have bought a 0.5 inch cutter which should make this way easier) and I am also going to try using a zinc electrode for the zinc plating, which should make things easier. I also want to see if I can get a better non-reactive conductive coating for the cell electrodes, since the conductive HDPE I am using has a quite significant resistance. Things are looking up though!